Since the mechanisms of spermatogenesis are conserved across all animal phyla, insights into the causes of human male infertility may come from studies in model organisms. During post-meiotic spermatogenesis in Drosophila, a cyst of 64 haploid spermatid nuclei is matured into an organized bundle of 64 mature spermatozoa. This maturation process, which includes nuclear shaping and elongation, construction of the axoneme, and spermatid individualization, in which each elongated spermatid is invested within its own plasma membrane, occurs within a spermatogenic cyst comprised of two somatic cyst cells. While these two cyst cells have been shown to play critical signaling roles during early spermatogenesis, their roles during post-meiotic spermatogenesis remain uncharacterized. This work seeks to uncover genes required in the somatic cyst cells for the post-meiotic maturation of spermatozoa. Using both P-element enhancer-detector lines and GFP-tagged protein traps, genes expressed in the cyst cells during late-stage spermatogenesis will be isolated. Subsequently, the corresponding genes will be uncovered by isolating neighboring genetic units from candidate insertions and mutations will be obtained. In a complimentary approach, novel mutations in appropriately expressed genes will be generated by P-element mobilization. Newly obtained mutants will be screened for late-stage spermatogenic defects using light microscopy. Phenotypically interesting mutations will be further characterized using known molecular markers and epi-flourescence microscopy. Candidate somatic regulators of post-meiotic spermatogenesis will be sequenced and related genes will be identified in other species. Since insect and mammalian spermatogenesis are functionally similar, identification of somatic regulators of spermatid maturation in Drosophila may help uncover interactions between the somatic Sertoli cells and the germline in the mammalian testis, thus providing insight into somatic regulation of mammalian spermatid morphogenesis, and ultimately, into the mechanisms governing human male fertility.